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Address Lookup In Packet Data Communications Link, Using Hashing And Content-addressable Memory - Patent 5414704


This invention relates to address translation as used in packet data communications, and more particularly to a way of doing source and destination address lookups in such a system, using a combination of hashing, binary search, and CAM lookup.In a packet data communication network of the Ethernet, token ring, or FDDI type, for example, there is usually the need to translate addresses. Some protocols or systems specify a 48-bit source and destination address so that a globally uniqueaddress is provided. However, for efficient use of resources at a local segment of a large network, it is advantageous to use smaller address fields instead of 48-bit addresses, for efficiency in bit-count of messages as well as efficiency in processingand storage. For this reason, while the 48-bit addresses are carried in the packet throughout its lifetime, shorter addresses are generated for local routing and processing. Thus, a translation mechanism must be provided to allow switching betweenglobal and local addresses. Examples of prior address translation methods used in packet dam communications networks are disclosed in U.S. Pat. Nos. 4,933,937, 5,027,350, and 5,136,580.A typical translation method employs a database of addresses in one format, with indexing into the database using the addresses of the other format. Hashing is often used in a method of this type to shorten size of the addresses needed, and toprovide more heavily populated data structures (reduce memory requirements). A binary search engine is also commonly used for such address lookups. Content addressable memories are a third technique for solving the search requirement.An address database of, for example, 16K addresses (requiring a 14-bit address to enter) would require a worst case of fourteen reads in a straightforward binary search. This is prohibitive from a performance standpoint, because the devicereceiving the packet with the address must process packets at a very high rate to keep current with

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